This proposal will attempt to identify expression-based markers for breast cancer and develop assays using these markers to detect circulating breast cancer cells. Our application has been developed as a collaborative effort between the Duke Breast Cancer Program and Abbott Diagnostic Breast Cancer Venture Group and will utilize the considerable strengths of each institution. The basic approach for marker identification takes advantage of ongoing (and separately funded) efforts using powerful new genomics tools. At Duke, SAGE (serial analysis of gene expression) libraries of breast cancer and normal breast epithelium are being constructed as part of CGAP. Also at Duke, a series of 20 primary breast cancers will be analyzed using high-density Affymetrix expression chips (30,000 genes/ESTs will be sampled). Finally, at Abbott Diagnostics, markers have already been identified by using the Incyte Corporation expression libraries. These three sources of expression data will be compared electronically and the most promising markers that are expressed at high levels in a tissue or cancer specific manner will be identified. After the in silico analysis, the lead markers will be put through a validation algorithm that will include northern blotting, multiple tissue RNA dot blots, RT-PCR, and in situ cytohybridization. A panel of breast cell lines and a series of 50 primary breast cancers will be used to determine how commonly expressed these markers are in the target tissue. After these initial validation steps, a series of reconstitution experiments will be performed by spiking normal whole blood with known numbers of breast cancer cells, derived from cell lines and malignant effusions. Epithelial cells will be purified using magnetic bead/antibody technology and the resulting purified samples will be analyzed by flow cytometry, histology, and quantitative RT-PCR. Towards increasing sensitivity, a separate and parallel screen will focus on markers that are highly overexpressed in a subset of breast cancers. PCR multiplexing approaches will be explored in this context and will also include using telomerase gene (TERT) expression as a cancer specific marker in conjunction with other lead markers. Antibodies to the most promising candidates will be developed, particularly for gene products that may be secreted or presented on the cell surface. Finally, a collection of whole blood and serum samples will be obtained from women newly diagnosed with breast cancer at Duke. This bank will constitute a primary validation set for markers and approaches that have proven to be the most robust.